System and method for permitting out-of-order execution of load and store instructions

ABSTRACT

In a load/store unit within a microprocessor, load and store instructions are executed out of order. The load and store instructions are assigned tags in a predetermined manner, and then assigned to load and store reorder queues for keeping track of the program order of the load and store instructions. Then when new load or store instructions are issued, the new load or store instructions are compared to entries within the load and store reorder queues to detect out of order problems.

TECHNICAL FIELD

The present invention relates in general to data processing systems, and in particular, to the execution of out-of-order load and store instructions in a processor.

BACKGROUND INFORMATION

In order to increase the operating speed of microprocessors, architectures have been designed and implemented that allow for the out-of-order execution of instructions within the microprocessor. An advantage of out-of-order execution of instructions is that it allows load miss latencies to be hidden while useful work is being performed. However, traditionally, load and store instructions have not been executed out of order because of the very nature of their purpose. For example, if a store instruction is scheduled to be executed in program order prior to a load instruction, but the processor executes these two instructions out of order so that the load instruction is executed prior to the store instruction, and these two instructions are referring to the same memory space, there is a likelihood that the load instruction will load incorrect, or old, data since the store instruction was not permitted to complete prior to the load instruction.

Nevertheless, techniques have been implemented to attempt to execute load and store instructions out of order. However, such techniques have often required too many processor cycles to execute. As microprocessor speeds continually increase, there is a need in the art for an ability to execute in parallel such load and store instructions and to correct for such problems as described above in a more efficient and faster manner.

SUMMARY OF THE INVENTION

The present invention provides a mechanism to allow out-of-order load and store execution and a means to recover from problems which occur from such execution in an efficient manner.

The present invention addresses the foregoing need by tagging the load and store instructions and then maintaining entries in separate queues for the load and store instructions, in conformance with the assigned tags.

At instruction dispatch, each store instruction is assigned an STAG (store tag) and LTAG (load tag), where the STAG is incremented by a preceding store instruction (in program order) and the LTAG is incremented by preceding load instructions. Each load instruction is assigned an LTAG and STAG, where the LTAG is incremented by a preceding load instruction (in program order). Addresses are queued in a load reorder queue and a store reorder queue in position relative to their LTAG (STAG). Conflicts can then be detected since the relative program order is known at address generation time.

The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a data processing system configured in accordance with the present invention;

FIG. 2 illustrates a load/store unit configured in accordance with the present invention;

FIG. 3 illustrates a tagging method in accordance with the present invention;

FIG. 4 illustrates ordering of load and store instructions in a load reorder queue and a store reorder queue, respectively; and

FIG. 5 illustrates a flow diagram for flushing instructions in accordance with the present invention.

DETAILED DESCRIPTION

The present invention provides for the out-of-order execution of load and store instructions, whereby load instructions are executed speculatively to hide load miss latencies. A load reorder queue is utilized to catch the instances where a younger load instruction is executed before an older store instruction, whereby the two instructions have an address byte overlap. This store-hit-load detection is performed by the load reorder queue using the tagging method described herein.

In the following description, numerous specific details are set forth such as specific word or byte lengths, etc. to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details. In other instances, well-known circuits have been shown in block diagram form in order not to obscure the present invention in unnecessary detail. For the most part, details concerning timing considerations and the like have been omitted inasmuch as such details are not necessary to obtain a complete understanding of the present invention and are within the skills of persons of ordinary skill in the relevant art.

Refer now to the drawings wherein depicted elements are not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views.

A representative hardware environment for practicing the present invention is depicted in FIG. 1, which illustrates a typical hardware configuration of workstation 113 in accordance with the subject invention having central processing unit (CPU) 110, and a number of other units interconnected via system bus 112. CPU 110 embodies the load/store unit 201 of the present invention as described below. Workstation 113 includes random access memory (RAM) 114, read only memory (ROM) 116, and input/output (I/O) adapter 118 for connecting peripheral devices such as disk units 120 and tape drives 140 to bus 112, user interface adapter 122 for connecting keyboard 124, mouse 126, and/or other user interface devices such as a touch screen device (not shown) to bus 112, communication adapter 134 for connecting workstation 113 to a data processing network, and display adapter 136 for connecting bus 112 to display device 138. CPU 110 may include other circuitry (other than load/store unit 201) not shown herein, which will include circuitry commonly found within a microprocessor, e.g., execution unit, bus interface unit, arithmetic logic unit, etc. CPU 110 may also reside on a single integrated circuit.

FIG. 2 illustrates load/store (L/S) unit 201 configured in accordance with the present invention. L/S unit 201 is located within CPU 110, which may be configured in accordance with typical microprocessor architectures.

L/S unit 201 has two pipelines so that two load or store instructions can be issued per machine cycle. Registers 202-205 receive instructions from fixed point units (FXUs) 0 and 1 (not shown) in a manner well-known in the art. 64-bit adder 206 adds operands received from registers 202 and 203, while 64-bit adder 207 adds operands from registers 204 and 205 to produces a pair of 64-bit effective addresses. These effective addresses are outputted to registers 208 and 209, respectively. Registers 208 and 209 capture the effective addresses (EA). They then both feed LMQ 218, LRQ 220 and SRQ 222, which all need a portion of the EA in addition to the real address from registers 216 and 217 to perform address checking. Additionally, the effective addresses are decoded to access tag arrays 210 and 211 to determine if there is a hit or a miss within L1 cache 236. If there is a miss, then the addresses are passed through registers 212 and 213 and sent to the L2 cache (not shown).

Furthermore, the effective addresses are sent from adders 206 and 207 to be decoded and to access the effective real address translator (ERAT) arrays 214 and 215, respectively, which output translated addresses through registers 216 and 217.

Further, the effective addresses from adders 206 and 207 access the L1 cache 236 for the load operation after being decoded by the decoders within the L1 cache 236. If there is a hit in the L1 cache 236, then the data is read out of the L1 cache 236 into registers 237, 238, and formatted by formatters 240, 241, and returned on the result bus to be sent to a register file (RegFile) (not shown). The cache line read out of L1 cache 236 is also returned into the registers 202-205 for operations that are dependent on the result as an operand.

Essentially, the three cycles performed within L/S unit 201 are the execute cycle (where the addition is performed), the access cycle (where access to the arrays is performed), and the result cycle (where the formatting and forwarding of data is performed).

If there is a miss in the cache, the request is then sent down to the L2 cache (not shown). The load miss queue (LMQ) 218 waits for the load data to come back from the L2 cache (not shown). The data associated with that cache line is loaded into the L1 cache 236.

These load operations can be performed speculatively and out of order. Store instructions are also executed out of order. Store instructions are run through the translation operation in translators 214, 215, then inserted into the store data queue (SDQ) 221 for storage into the L1 cache 236 after the instructions have been completed. Therefore, store instructions are executed out of order, but written into the L1 cache 236 in order.

The store reorder queue (SRQ) 222 keeps track of store instructions that have been executed. SRQ 222 maintains the store instructions in the queue and determines when the data is available in the store data queue (SDQ) 221 and when the store instruction is next to complete. The store to the L1 cache 236 is then completed.

Many of the registers 223, 225-229, and 237-238, are utilized for timing.

Cache lines within the L1 cache 236 are accessed based on the effective address of the cache line. The RA tag array 233 keeps track of where in the L1 cache 236 a cache line was written. The format block 231 takes the data from the SDQ 221 and rotates it properly to write into the correct byte positions in the L1 cache 236 upon execution of the store instruction. Rotate blocks 224 and 230 are utilized for store forwarding. Therefore, if there is a store instruction that is sitting in the store queue and has not been written into the queue yet because it is not next to complete, and then a younger load instruction is received that needs that data, the data will be forwarded to the load instruction being executed.

Rotate block 239 is utilized to rotate data received from the L2 cache (not shown) in response to an L1 cache miss, for forwarding the data from the L2 cache on to the result bus for forwarding to the proper register file.

Block 219 contains a number of special purpose registers to store data as a result of special purpose register instructions and read data from these registers so they get into the normal pipeline.

Register 235 is implemented for timing purposes to stage data from the L2 cache (not shown). Format blocks 240 and 241 format (or shift) cache data into the proper byte positions for the load result to the register file.

Referring next to FIG. 3, there is illustrated a diagram of a method for assigning tags to load and store instructions in accordance with the present invention. Instructions are received from the instruction cache 270 by the instruction dispatch unit 271, which assigns tags to the load and store instructions as described herein. The instructions are then temporarily stored in the instruction queue 272. Tags 273 and 274 are pipeline registers used for timing purposes. 16 instructions in blocks of 4 (blocks 301-304) are analyzed at a time for the load/store tagging method of the present invention. Each block 301-304 is assigned a group tag (GTAG). Each store instruction is assigned an STAG and an LTAG. The STAG is incremented by a preceding store instruction (in program order), and the LTAG is incremented by preceding load instructions. Similarly, the LTAG is incremented by a preceding load.

For example, the program order of the store instructions are S1, S2, S3, and S4. Store instruction S1 has an STAG of 9. The next store instruction S2 is then assigned an STAG of 10. The next store instruction S3 has an STAG of L1, and then the STAG is incremented to a 12 for the next store instruction S4. The load instructions L1, L2, L3, and L4 (in program order) are assigned the STAGs of the previous store instruction. Therefore, load instruction L1 receives an STAG of 10, which is the same STAG as the preceding store instruction S2. Load instruction L2 receives an STAG of 11, which is the same STAG as the preceding store instruction S3. Load instruction L3 receives an STAG of 12, which is the same STAG as the preceding store instruction S4. Load instruction L4 also receives an STAG of 12, since the STAG that immediately precedes the load instruction L4 is still the store instruction S4 having an STAG of 12.

The LTAGs for the store instructions are incremented based on the LTAG of a preceding load instruction. As a result, the LTAG for store instruction S3 is incremented to 6 because the LTAG for the preceding load instruction L1 is 5. The LTAGs are not incremented until the next store instruction S4 which is assigned an LTAG of 7 based on the previous LTAG of 6 for the load instruction L2. LTAGs are also incremented by a preceding load instruction. Therefore, the LTAG for load instruction L4 is assigned an 8 because the LTAG for the preceding load instruction L3 is a 7.

Referring next to FIG. 4, the addresses for the load store unit 0 (ls0_address) and the load store unit 1 (ls 1_address) are queued in the load reorder queue 220 and the store reorder queue 222 in position relative to their LTAG (STAG). The pointers sc_comp_ltag and sc_comp_stag for the LRQ 220 and SRQ 222, respectively, indicate the last load or store instruction to complete, respectively.

As an example, load instruction L1 is placed in a position in LRQ 220 corresponding to its LTAG assignment of 5.

Referring next to FIG. 5, when a new store instruction is issued for execution within L/S unit 201, it will be assigned a specified LTAG (step 501) and STAG. This store instruction with its specified LTAG is then compared to the LTAGs within LRQ 220. Beginning with the specified LTAG of the store instruction, the present invention searches forward within the L,RQ 220 up to the sc_comp_ltag pointer for a younger load instruction that has executed and belongs to a cache line that has been invalidated (step 502). If such a younger load instruction is found, then in step 503, this younger load instruction is flushed for re-execution. In step 504, the store instruction that has been issued is then executed.

The present invention assigns each load instruction with a load tag that indicates its program order. The LRQ 220 keeps track of all of the outstanding load instructions in the machine. Invalidations are also run past the LRQ 220 and the SRQ 222. Thereafter, if a load or store instruction is then received that has a byte overlap with an entry in the queue that has been invalidated, then the present invention flushes the younger load instruction for re-execution. The present invention is able to do this with a quick CAM (content addressable memory) address compare of entries in a queue (LRQ 220, SRQ 222), as opposed to relying upon a plurality of state machines to keep track of load and store instructions.

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. 

What is claimed is:
 1. A method for assigning tags to load and store instructions, comprising the steps of: assigning first tags to each of the load and store instructions, wherein the first tags are incremented for each store instruction in program order; and assigning second tags to each of the load and store instructions, wherein the second tags are incremented for each store instruction in program order that is preceded by a load instruction.
 2. The method as recited in claim 1, wherein the second tags are not incremented for a store instruction that is preceded by a store instruction.
 3. The method as recited in claim 1, wherein the second tags are incremented for each load instruction in program order that is preceded by a load instruction.
 4. The method as recited in claim 1, wherein when the second tags are incremented for each store instruction in program order that is preceded by a load instruction, there is no other store instruction between the each store instruction and a preceding load instruction in program order.
 5. The method as recited in claim 2, wherein when the second tags are not incremented for a store instruction that is preceded by a store instruction, there is no other store instruction between the store instruction and its preceding store instruction in program order.
 6. The method as recited in claim 3, wherein when the second tags are incremented for each load instruction in program order that is preceded by a load instruction, there is no other load instruction between the each load instruction and its preceding load instruction in program order.
 7. The method as recited in claim 1, wherein the load and store instructions are arranged in blocks of a plurality of such instructions, wherein a third tag is assigned to each block.
 8. The method as recited in claim 1, further comprising the steps of: storing addresses of the load instructions in a load reorder queue in an order in accordance to the assigned second tags; and storing addresses of the store instructions in a store reorder queue in an order in accordance to the assigned first tags.
 9. A method for assigning tags to load and store instructions in a load/store unit, comprising the steps of: assigning a store tag N and a load tag M to a first store instruction; if a next load or store instruction in program order from the first store instruction is a second store instruction, then assigning a store tag N+1 and a load tag M to the second store instruction; and if the next load or store instruction in program order from the first store instruction is a first load instruction, then assigning a store tag N and a load tag M to the first load instruction.
 10. The method as recited in claim 9, further comprising the steps of: if a next load or store instruction in program order from the first load instruction is a third store instruction, then assigning a store tag N+1 and a load tag M+1 to the third store instruction; and if the next load or store instruction in program order from the first store instruction is a second load instruction, then assigning a store tag N and a load tag M+1 to the second load instruction.
 11. A processor comprising: a load/store unit further comprising: circuitry for issuing for execution load and store instructions in an out of order manner; circuitry for assigning first tags to each of the load and store instructions, wherein the first tags are incremented for each store instruction in program order; and circuitry for assigning second tags to each of the load and store instructions, wherein the second tags are incremented for each store instruction in program order that is preceded by a load instruction.
 12. The processor as recited in claim 11, wherein the second tags arc not incremented for a store instruction that is preceded by a store instruction.
 13. The processor as recited in claim 12, wherein the second tags are incremented for each load instruction in program order that is preceded by a load instruction.
 14. The processor as recited in claim 13, wherein when the second tags are incremented for each store instruction in program order that is preceded by a load instruction, there is no other store instruction between the each store instruction and a preceding load instruction in program order.
 15. The processor as recited in claim 14, wherein when the second tags are not incremented for a store instruction that is preceded by a store instruction, there is no second store instruction between the store instruction and its preceding store instruction in program order.
 16. The processor as recited in claim 15, wherein when the second tags are incremented for each load instruction in program order that is preceded by a load instruction, there is no second load instruction between the each load instruction and its preceding load instruction in program order.
 17. The processor as recited in claim 16, wherein the load and store instructions are arranged in blocks of a plurality of such instructions, wherein a third tag is assigned to each block.
 18. The processor as recited in claim 11, further comprising: a load reorder queue for storing addresses of the load instructions in an order in accordance to the assigned second tags.
 19. A data processing system comprising: a processor coupled by a bus system to a memory device, a storage device, an output device, and an input device, wherein the processor includes a plurality of execution units including a load/store unit comprising: circuitry for assigning a store tag N and a load tag M to a first store instruction; if a next load or store instruction in program order from the first store instruction is a second store instruction, then circuitry for assigning a store tag N+1 and a load tag M to the second store instruction; and if the next load or store instruction in program order from the first store instruction is a first load instruction, then circuitry for assigning a store tag N and a load tag M to the first load instruction.
 20. The system as recited in claim 19, further comprising: if a next load or store instruction in program order from the first load instruction is a third store instruction, then circuitry for assigning a store tag N+1 and a load tag M+1 to the third store instruction; and if the next load or store instruction in program order from the first store instruction is a second load instruction, then circuitry for assigning a store tag N and a load tag M+1 to the second load instruction.
 21. The system as recited in claim 19, further comprising: circuitry for executing the first load instruction; circuitry for executing the first store instruction after the first load instruction; a load reorder queue for detecting that the first load instruction and the first store instruction have an address byte overlap; and circuitry for flushing the first load instruction in response to the load reorder queue detecting that the first load instruction and the first store instruction have the address byte overlap. 